What happens if a PLC is connected to an ungrounded transformer with floating voltage, if power is turned off and back on?
If a PLC is powered from an ungrounded/floating transformer secondary, turning the power OFF and back ON can make an intermittent problem disappear, reappear, or change even though the transformer itself is producing the correct line-to-line voltage.
The key is that the PLC needs two things: the correct supply voltage between its power terminals, and a reasonably controlled voltage relationship between its circuitry, chassis, shields, I/O commons, and ground.
Suppose the transformer secondary is 120 VAC:
ISOLATION TRANSFORMER X1 o───────────────o PLC L 120 VAC X2 o───────────────o PLC N/L2 Neither X1 nor X2 bonded to ground
The PLC may see a perfectly good:
X1 → X2 = 120 VAC
and operate normally.
But because the secondary is floating, you might see:
X1 → Ground = 35 VAC X2 → Ground = 85 VAC
or:
X1 → Ground = 60 VAC X2 → Ground = 60 VAC
Those readings can change with wiring capacitance, filters, connected equipment, VFDs, power supplies, and the PLC's own EMI suppression components.
What happens when you turn power OFF
Inside a PLC power supply are capacitors and EMI-filter components. When you remove power:
Transformer OFF ↓ PLC input voltage collapses ↓ PLC power-supply capacitors discharge ↓ CPU resets ↓ Internal reference voltages disappear
At the same time, the floating secondary can lose whatever temporary ground reference it had acquired through connected equipment.
When power comes back:
Transformer energized ↓ Secondary voltage returns ↓ PLC power supply charges ↓ PLC CPU performs startup/reset ↓ I/O modules initialize ↓ Communications initialize ↓ Floating ground reference establishes itself again
That new reference does not necessarily settle exactly where it was before.
This can explain equipment that:
“Wouldn't run, but we shut the power off and turned it back on and now it works.”
The power cycle may have reset the PLC, but it may also have temporarily changed the common-mode or floating-voltage condition.
Where the trouble usually appears
The PLC CPU itself may run perfectly well because it mainly cares about the voltage between its supply terminals. Problems often show up instead in the I/O and communications.
For example:
Floating transformer │ ├── PLC │ ├── 24 VDC power supply │ ├── sensors │ ├── analog transmitters │ └── communication devices
If different devices acquire different references to earth or machine ground, you can get:
false digital inputs
inputs that fail to turn ON
analog signals that jump or drift
unexplained PLC I/O faults
communication dropouts
Ethernet/shield noise problems
intermittent module faults
a PLC that operates after a reboot but faults again later
A particularly important case is the 24-VDC supply.
Imagine:
120 VAC floating secondary │ ▼ 24 VDC POWER SUPPLY +24V 0V │ │ │ └──── PLC DC Common │ └────────────── PLC / Sensors
If the power supply's DC common is also floating, its voltage to machine ground may wander because of internal filter capacitance:
+24 V → Ground = perhaps +70 V AC common-mode 0 V → Ground = perhaps +45 V AC common-mode
A high-impedance meter may show strange readings even though:
+24 V → 0 V = 24.0 VDC
The PLC may continue functioning until some other device establishes an unintended ground path.
For instance:
PLC 0V floating | +──────── sensor | +──── machine frame
Now the sensor unintentionally becomes the grounding path.
That can create intermittent behavior.
Why a restart can appear to "fix" it
A power cycle can do several things simultaneously:
Reset the PLC CPU.
Reset I/O modules.
Reset electronic overloads or communication adapters.
Discharge internal capacitors.
Remove an accumulated common-mode charge.
Re-establish the floating system at a different voltage relative to earth.
Temporarily clear leakage or capacitive effects.
So a successful restart does not prove the PLC was the problem.
That is a very important troubleshooting lesson.
What I would measure
For a 120-V transformer feeding a PLC, I would check all three:
X1 → X2 X1 → Ground X2 → Ground
Then check the PLC supply:
PLC L1 → L2 PLC L1 → Ground PLC L2 → Ground
If there is a 24-VDC supply:
+24 V → 0 V +24 V → Ground 0 V → Ground
Do these measurements before the power cycle if the fault is present, because once you shut the equipment off you may destroy the evidence.
That is one of the biggest mistakes in industrial troubleshooting:
FAULT OCCURS ↓ Technician immediately cycles power ↓ Fault disappears ↓ Evidence is lost
A better sequence is:
FAULT OCCURS ↓ DO NOT RESET YET ↓ Check transformer voltage ↓ Check voltage-to-ground ↓ Check PLC LEDs / fault codes ↓ Check 24 VDC and DC common-to-ground ↓ Check I/O status ↓ Check communication faults ↓ Record findings ↓ Then reset if appropriate
One final distinction matters: a deliberately ungrounded control transformer isn't automatically wrong. Some control systems are intentionally isolated. But the entire grounding/reference strategy must be engineered correctly. If the PLC manufacturer, control-system design, or applicable electrical rules call for one secondary conductor or the DC common to be bonded, leaving it floating can create exactly the sort of intermittent troubleshooting nightmare you're describing.
For a Shepherd Manuals troubleshooting guide, I would make this a dedicated page titled “PLC Works After Power Cycle: Check for Floating Control Power Before Replacing the PLC.” That's a very practical field lesson.
Is it possible that the plc with work for perhaps years then lose its reference and not work again until the transformer itself X2 is bonded to ground
Yes — that is possible, but I would describe the failure a little differently.
A PLC system can operate for years from a floating transformer secondary because the PLC still receives the correct X1-to-X2 voltage. Over time, however, leakage paths, capacitive coupling, EMI filters, connected equipment, contamination, aging insulation, VFD noise, communication shields, or a developing ground fault can change the secondary’s relationship to earth ground enough that the PLC power supply or I/O circuitry begins having common-mode/reference problems.
For example:
CONTROL TRANSFORMER 120 V SECONDARY X1 o─────────────── PLC L1 120 VAC X2 o─────────────── PLC L2 NO GROUND BOND
The PLC may operate for years because:
X1 → X2 = 120 V
even though:
X1 → Ground = unpredictable X2 → Ground = unpredictable
Then something in the system changes.
Years of normal operation ↓ Insulation / filters / wiring / devices change ↓ Unintended leakage path develops ↓ Floating secondary shifts relative to ground ↓ Common-mode voltage increases ↓ PLC power supply / I/O / communications become unstable
At that point you might measure something like:
X1 → X2 = 120 VAC ✓ X1 → Ground = 105 VAC X2 → Ground = 45 VAC
Those ground readings aren't necessarily mathematically complementary because the meter is interacting with high-impedance leakage and capacitive paths.
Now suppose you intentionally bond X2:
X1 o────────────── PLC L1 120 VAC X2 o────────────── PLC L2 | | ⏚ GROUND
The references become:
X1 → X2 ≈ 120 VAC X1 → Ground ≈ 120 VAC X2 → Ground ≈ 0 VAC
The PLC power supply now has a stable common-mode reference, and the equipment may immediately operate normally again.
But this is the important diagnostic point
I would not conclude simply that “the transformer lost its ground reference.” If it was intentionally floating, it never had a fixed ground reference to lose.
A better diagnosis is:
The system previously tolerated a floating secondary, but a change in leakage, capacitance, grounding, connected equipment, or insulation caused the floating system's common-mode voltage to become incompatible with reliable PLC operation. Bonding X2 established a defined reference and stabilized the system.
And I would investigate why it changed after years of operation, especially if bonding X2 suddenly fixes the machine.
Check:
X1–X2 voltage before making the bond.
X1–ground and X2–ground with a high-impedance DMM.
Repeat ground measurements using an appropriate LoZ method.
PLC power-supply input.
24 VDC + to 0 V.
24 VDC 0 V to ground.
Insulation/leakage from X1 and X2 to ground.
Surge suppressors/MOVs.
EMI/RFI filters.
VFDs and servo drives connected to the control system.
Shields and communication grounding.
Any recently replaced sensor, power supply, PLC module, or other device.
Moisture, carbon tracking, contamination, or deteriorated wiring.
One scenario I would be particularly suspicious of is:
PLC | FLOATING 120 VAC → 24 VDC SUPPLY | 0 V | unintended leakage | MACHINE FRAME
For years that unintended path may have effectively provided enough reference for the equipment to behave normally. When that path changes or disappears, the PLC system starts behaving strangely.
Then bonding X2 provides the stable reference the system was inadvertently getting elsewhere.
So yes, your proposed real-world sequence is entirely plausible:
PLC operates for years ↓ Something changes in leakage/reference path ↓ PLC becomes unreliable or will not operate ↓ Power cycling no longer fixes it ↓ X2 intentionally bonded correctly to ground ↓ Stable voltage reference established ↓ PLC operates normally
If I encountered that in a plant, though, I would treat the fact that grounding X2 cured a years-old intermittent failure as a major diagnostic clue, not merely call the transformer fixed. I'd look for the electrical change that caused the floating system to stop behaving satisfactorily before declaring the job complete.
Contact: angie@shepherdmanuals with questions
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